Touch screens are now ubiquitous and used as the input and display interface at, for example, automatic teller machines, gambling machines in casinos, cash registers, and the like. Touch screen panels generally comprise an insulative (e.g., glass) substrate and a resistive layer disposed on the insulative substrate. A pattern of conductive edge electrodes are then formed on the edges of the resistive layer. The conductive electrodes form orthogonal electric fields in the X and Y directions across the resistive layer. Contact of a finger or stylus on the active area of the panel then causes the generation of a signal that is representative of the X and Y coordinates of the location of the finger or the stylus with respect to the substrate. In this way, the associated touch panel circuitry connected to the touch panel by wires or wiring traces can ascertain where the touch occurred on the substrate. Typically, a computer program generates an option to the user (e.g., “press here for ‘yes’ and press here for ‘no’”) on a monitor underneath the touch screen panel and the conductive edge electrode pattern assists in detecting which option was chosen when the touch panel was used by the user.
In the prior art, a resistive layer (e.g., tin antimony oxide) is sputtered onto a glass substrate. The conductive edge electrodes and wire traces are then deposited on the resistive layer about the periphery of the panel using a thick film paste. A SiO2 transparent hard coating is then applied to the panel over the conductive edge electrodes and wire traces to protect the panel during use.
Because of the thickness of the edge electrodes and wire traces, however, the hard coat is not planar and instead rises up at the edges of the panel causing a cosmetic defect in that color variations are present at the edges of the panel. These unacceptable color variations are a major yield issue of capacitive touch screen panels incorporating SiO2 transparent hard coatings applied by a wet chemical processes when the liquid hard coat material dams and drains around the edge electrodes and wire traces.
Furthermore, cracking and islanding of the thick-film material causes functional failures and is an additional major yield issue in capacitive touch screen manufacturing. This problem is caused by a chemical interaction between the thick-film of the edge electrodes and wire traces and the SiO2 transparent hard coating and by mechanical stress on the thick-film during densification of the SiO2 hard coating network.
If, on the other hand, the SiO2 transparent hard coat material is deposited before the edge electrodes and the wire traces so that the hard coat is under the edge electrodes and wire traces to eliminate the color variation problems, the hard coat material prevents the establishment of the correct electrical connection between the edge electrodes and the wire traces with the resistive coating.